Synthesis and characterization of nanocatalyst Cu2+/mesoporous carbon for amidation reactions of alcohols

In this research, mesoporous carbon (MC) with high efficiency (0.65 g yield from 1.0 g MCM-41 and 1.25 g sucrose) was successfully prepared by adding carbon precursor (sucrose) in a single step with ultrasonic waves, which reduces time and energy cost. Then, the Cu2+/Mesoporous carbon nanocatalyst (Cu2+/MC) was synthesized by adding Cu(NO3)2 in a single step and applied as a catalyst in amidation reactions of alcohols. Also, Cu2+/MC was characterized using different spectroscopic methods and techniques, including Fourier transform infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM), N2 adsorption analysis (BET), X-ray diffraction analysis (XRD), Energy Dispersive X-ray (EDX), and Thermogravimetric Analysis (TGA). Moreover, to show the catalytic merits of Cu2+/MC, various primary and secondary amines and ammonium salts were applied in the amidation of alcohols. Easy synthesis method, recyclability, excellent yields (80–93%), and simple work-up are some noticeable strengths of using Cu2+/MC as a catalyst in this reaction.

The procedure for the synthesis of MC. For this purpose, synthesized MCM-41 (1.0 g) was added slowly to a mixture of water (5.0 mL), sulfuric acid (1.5 mL), and sucrose (1.25 g) and stirred until a homogeneous solution has obtained. Afterward, the mixture was subjected to ultrasonic waves for 3 h until the sucrose precursor was completely inserted into the pores of MCM-41. Then, the mixture was placed in a 100 °C oven for 6 h to dry completely. At this stage, the color of the compound was changed to burnt brown or black. Next, the product was placed in the furnace under a nitrogen atmosphere at 800 °C by rate of 10 °C/min. The color of the obtained powder after the furnace changed to black. For removing the MCM-41, the obtained black powder was poured into a solution of ammonium bifluoride salt (40 mL, 4 M) and stirred. After 24 h, the black powder was separated by centrifugation, washed with water and ethanol, and dried in an 80 °C oven to obtain mesoporous carbon. The graphical scheme of this procedure is shown in Fig. 3.   was added to a solution of distilled water (10 mL) and Cu(NO 3 ) 2 (1.0 g) and stirred until the homogeneous mixture was obtained. The mixture is then subjected to ultrasonic waves for 2 h so that the coppers are placed into the holes of the MC. Afterward, the mixture was passed through the filter, washed with water and ethanol, and placed in an 80 °C oven to dry. The resulting black powder is a carbon composite of Cu 2+ /MC. The graphical scheme of this procedure is shown in Fig. 4.
General procedure for direct amidation of benzyl alcohols.   www.nature.com/scientificreports/

Result and discussion
In this project, ultrasonic waves were used to add sucrose to the silica template in one step. The yield of MC for one-step synthesizing by reported procedures is about 0.35 g, while in this project, it is about 0.65 g. FT-IR spectroscopy was used to investigate the synthesis of Cu 2+ /MC and approve the presence of expected functional groups. The FT-IR spectrum of the Cu 2+ /MC is shown in Fig. 5. The peak around 3430 cm −1 could be described as the vibrational stretching of -OH groups of adsorbed H 2 O in the catalyst structure. The absorption peaks at about 1620 cm −1 are related to the tensile vibration of the C=C groups of carbon rings 37 . The peaks around 1460 cm −1 and 1375 cm −1 were characterized as bending vibrations of CH 3 and CH 3 -CH 2 groups, respectively. Also, there are the oop C-H bending vibrations around 1034 cm −1 . The tensile vibration of the C-H groups corresponding to SP 3 carbons appears at approximately 2860 cm −1 and 2920 cm −1 . The peak at 2360 cm −1 is related to the absorption of CO 2 by the device and not relevant to the Cu 2+ /MC 38,39 .
Energy Dispersive X-Ray analysis was performed to determine the elements in the MC and Cu 2+ /MC composition. As shown in Fig. 6, the presence of essential atoms, such as carbon, oxygen, and copper, in the fabricated MC and Cu 2+ /MC structure has also been verified. A small amount of silica is observed in the EDX analysis, which is because, after repeated washing, a small amount of silicon remains in the structure and is not completely removed 40,41 . A very high percentage of carbon indicates the successful synthesis of MC. Also, there is a low amount of oxygen in the structure assigned to the absorption of water at the surface of the MC pores. The Cu peak indicates loaded copper in pore channels of MC, where the catalyst's loaded Cu (II) is 3.5 wt% by ICP/OES analysis. Moreover, the elemental mapping of Cu 2+ /MC was taken to indicate the dispersity of the Cu (II) in the mesoporous carbon structure (Fig. 7).
The prepared MC and Cu 2+ /MC was crystallographically measured by wide-angle XRD Spectra in the range of 5 • to 90 • and shown in Fig. 8. It has two distinguish broad peaks at (002) and (101) (according to JCPDX index, No. 75-1621), which indicate the hexagonal graphitic structure of Cu 2+ /MC. There is no characteristic peak of Cu 2+ in the XRD pattern. It may be due to high dispersion and its small size 42 . Also, the distinguish broad peaks of MC are retained after synthesizing Cu 2+ /MC.
The morphology of MC ( Fig. 9, a and b) and Cu 2+ /MC (Fig. 9c, d) were observed by Field Emission Scanning Electron Microscopy. FE-SEM Images were shown that MCM-41 acts as a template and forms mesoporous carbon particles on its surface. The morphology of these particles is interconnected like a network structure. Cu 2+ species are uniformly distributed inside the structure of MC, and the structure is layered on top of each other, which XRD analysis is evidence of this claim.
To measure the surface area and pore size of the MC and Cu 2+ /MC, we used N 2 adsorption analysis and showed the results in Fig. 10. Also, the Table 1 is shown the MC and Cu 2+ /MC surface area (300.0553, 318.8345 m 2 /g), pore size (40.3784, 36.2053 Å), and pore volume (0.302893, 0.288588 cm 3 /g). Figure 9a, b show the N 2 adsorption and desorption isotherm diagrams of the MC and Cu 2+ /MC, respectively. These isotherms are similar to type (IV) isotherms that prove the mesoporous structure of synthesized compounds. According to Table 1, the pore size and the pore volume of Cu 2+ /MC have been reduced compared to MC, indicating copper trapping in the pores of mesoporous carbon. However, it can be observed that the surface area of Cu 2+ /MC has increased compared to MC due to the existence of copper on the crater of the pores.
Thermogravimetric analysis was used to investigate the thermal resistance of MC and Cu 2+ /MC, and the results were reported in Fig. 11a, b, respectively. According to Fig. 11a, the weight loss stage at an approximate temperature of 550 °C to 750 °C is due to the decomposition of the mesoporous carbon structure. The uniformity of the pattern before approximately 400 °C indicates the temperature resistance of Cu 2+ /MC. In addition, Cu 2+ / MC exhibits lower thermal stability than MC due to the distribution of Cu(II) in the MC structure. Also, there are no functional groups on the surface detected to degradation.  Table 2. For catalyzing tandem oxidative amidation of benzyl alcohols, benzylamine hydrochloride (1.0 mmol) and benzyl alcohol (1.5 mmol) were considered as a model reaction, and the intended product yield was obtained by the anti-solvent method (ethyl acetate, n-hexane). First, this reaction was investigated without the catalyst and applied TBHP and H 2 O 2 at 80 °C. According to the monitoring of TLC's reaction, no product was observed, which shows the catalyst's importance in this reaction ( Table 2, entries 1 and 2). Then, by adding the catalyst (10 mg) to the reaction mixture, in the presence of solvent and without oxidant, it was observed that the desired product was not formed ( Table 2, entry 3). It shows that adding the catalyst and oxidant both together would cause the reaction to proceed. In the following, in the presence of the catalyst, solvent, TBHP as an oxidant, and at room temperature, the reaction product's yield showed an efficiency of 23% (Table 1, entry 4). Increasing the reaction temperature from 25 to 80 °C improves the product's yield and leads to the reaction progressing and achieving higher efficiency. However, the 80 °C temperature is optimal, and above this temperature does not increase the yield and reduces product yields to 72% ( Table 2, entries 5 and 6). The reason is due to the effect of this factor on the oxidation of benzyl alcohol to benzoic acid, which happens at higher temperatures. Adding the amount of catalyst from 10 to 20 mg increased the yield of the intended product (  www.nature.com/scientificreports/ For evaluating the catalytic performance of Cu 2+ /MC nanocatalyst, several alcohols with electron-drawing and electron-donating groups with different types of amine hydrochloride salts were studied under optimized conditions in high to excellent yields (Table 4).
Also, the Cu 2+ /MC was evaluated with some other catalysts to compare its catalytic performance, shown in Table 5. By comparing the efficiency of the final product, this catalyst has shown outstanding performance in this reaction.   www.nature.com/scientificreports/  Figure 11. The TGA pattern of (a) MC and (b) Cu 2+ /MC. www.nature.com/scientificreports/

Mechanism
In this reaction, it is considered that the oxidation of alcohol and the formation of aldehyde as the intermediate proceeds through a radical mechanism in the presence of Cu 2+ /MC and TBHP. The obtained aldehyde enters the reaction with the free amine in the environment obtained by the deprotonation of its salt by calcium carbonate, and the carbinolamine (III) intermediate is obtained. Afterward, intermediate (IV) would obtain by the reaction of radical TBHP and carbinolamine intermediate. Finally, this intermediate is oxidized, and the desired product is obtained through the radical mechanism. The overall scheme of this mechanism is given in Fig. 12. Also, the reaction was performed in an O 2 atmosphere, and it was observed that no product was obtained. By approving this mechanism, Table 4 indicates that the withdrawing groups make the intermediate unstable, and the reaction yield would be increased. Also, the electron donating groups make the intermediate stable, and the yield would be decreased.

Reusability.
Reusability is one of the most critical factors of each catalytic system that highlights them as an efficient system due to the economic benefits and time-saving. For investigating catalyst recycling, the heterogeneous catalyst was first separated from the reaction mixture by filtration, washed with water and ethanol, and dried at 80 °C in an oven after each run to provide an opportunity for recycling experiments. It was observed that the catalyst could be reused at least five times with no significant reduction in its activity (Fig. 13). Also, EDX analysis was performed to determine the presence and stability of catalyst elements (Fig. 14). The result of the leaching of the Cu (II) after the recycling test was determined by ICP-OES analysis. The Cu (II) content in the synthesized catalyst was determined to be 3.5% before washing. This catalyst shows a content of 3.2% copper after reuse, confirming that the Cu (II) was not leached during the oxidative amidation reaction.

Conclusion
To conclude, we have successfully synthesized Cu 2+ /MC composite with ultrasonication, reduced the carbonization steps, and increased the yield of MC. This catalyst's performance has shown promising results in the direct amidation of various electron-donating and electron-withdrawing groups of alcohols and different benzylic and primary amine salts. The oxidant in this reaction was TBHP which is non-toxic and decomposes to water and tert-butanol. High yield, short reaction time, mild conditions, easy separation, and catalyst recyclability are the advantages of using Cu 2+ /MC composite as a catalyst in the direct amidation of alcohols reaction (Supplementary Information).  www.nature.com/scientificreports/   www.nature.com/scientificreports/

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request. www.nature.com/scientificreports/